Na channels are central to the generation of action potentials in all excitable cells such as neurons and myocytes. They play key roles in excitable tissue including brain, smooth muscles of the gastrointestinal tract, skeletal muscle, the peripheral nervous system, spinal cord and airway. As such they play key roles in a variety of disease states such as epilepsy (See, Moulard, B. and D. Bertrand (2002) “Epilepsy and sodium channel blockers” Expert Opin. Ther. Patents 12(1): 85-91)), pain (See, Waxman, S. G., S. Dib-Hajj, et al. (1999) “Sodium channels and pain” Proc Natl Acad Sci USA 96(14): 7635-9 and Waxman, S. G., T. R. Cummins, et al. (2000) “Voltage-gated sodium channels and the molecular pathogenesis of pain: a review” J Rehabil Res Dev 37(5): 517-28), myotonia (See, Meola, G. and V. Sansone (2000) “Therapy in myotonic disorders and in muscle channelopathies” Neurol Sci 21(5): S953-61 and Mankodi, A. and C. A. Thornton (2002) “Myotonic syndromes” Curr Opin Neurol 15(5): 545-52), ataxia (See, Meisler, M. H., J. A. Kearney, et al. (2002) “Mutations of voltage-gated sodium channels in movement disorders and epilepsy” Novartis Found Symp 241: 72-81), multiple sclerosis (See, Black, J. A., S. Dib-Hajj, et al. (2000) “Sensory neuron-specific sodium channel SNS is abnormally expressed in the brains of mice with experimental allergic encephalomyelitis and humans with multiple sclerosis” Proc Natl Acad Sci USA 97(21): 11598-602, and Renganathan, M., M. Gelderblom, et al. (2003) “Expression of Na(v)1.8 sodium channels perturbs the firing patterns of cerebellar purkinje cells” Brain Res 959(2): 235-42), irritable bowel (See, Su, X., R. E. Wachtel, et al. (1999) “Capsaicin sensitivity and voltage-gated sodium currents in colon sensory neurons from rat dorsal root ganglia” Am J Physiol 277(6 Pt 1): G1180-8, and Laird, J. M., V. Souslova, et al. (2002) “Deficits in visceral pain and referred hyperalgesia in Nav1.8 (SNS/PN3)-null mice” J Neurosci 22(19): 8352-6), urinary incontinence and visceral pain (SYoshimura, N., S. Seki, et al. (2001) “The involvement of the tetrodotoxin-resistant sodium channel Na(v)1.8 (PN3/SNS) in a rat model of visceral pain” J Neurosci 21(21): 8690-6), as well as an array of psychiatry dysfunctions such as anxiety and depression (See, Hurley, S. C. (2002) “Lamotrigine update and its use in mood disorders” Ann Pharmacother 36(5): 860-73).
Voltage gated Na channels comprise a gene family consisting of 9 different subtypes (NaV1.1-NaV1.9). As shown in Table 1, these subtypes show tissue specific localization and functional differences (See, Goldin, A. L. (2001) “Resurgence of sodium channel research” Annu Rev Physiol 63: 871-94). Three members of the gene family (NaV1.8, 1.9, 1.5) are resistant to block by the well-known Na channel blocker TTX, demonstrating subtype specificity within this gene family. Mutational analysis has identified glutamate 387 as a critical residue for TTX binding (See, Noda, M., H. Suzuki, et al. (1989) “A single point mutation confers tetrodotoxin and saxitoxin insensitivity on the sodium channel II” FEBS Lett 259(1): 213-6).
TABLE 1(Abbreviations: CNS = central nervous system,PNS = peripheral nervous sytem, DRG = dorsalroot ganglion, TG = Trigeminal ganglion):NaIsoformTissueTTX IC50IndicationsNaV1.1CNS, PNS10nMPain, Epilepsy,soma ofneurodegenerationneuronsNaV1.2CNS, high in10nMNeurodegenerationaxonsEpilepsyNaV1.3CNS, embryonic,15nMPain, Epilepsyinjured nervesNaV1.4Skeletal25nMMyotoniamuscleNaV1.5Heart2μMArrythmia,long QTNaV1.6CNS6nMPain, movementwidespread,disordersmost abuntantNaV1.7PNS, DRG,25nMPain,terminalsNeuroendocrineneuroendocrinedisordersNaV1.8PNS, small>50μMPainneurons inDRG & TGNaV1.9PNS, small1μMPainneurons inDRG & TG
In general, voltage-gated sodium channels (NaVs) are responsible for initiating the rapid upstroke of action potentials in excitable tissue in nervous system, which transmit the electrical signals that compose and encode normal and aberrant pain sensations. Antagonists of NaV channels can attenuate these pain signals and are useful for treating a variety of pain conditions, including but not limited to acute, chronic, inflammatory, and neuropathic pain. Known NaV antagonists, such as TTX, lidocaine (See, Mao, J. and L. L. Chen (2000) “Systemic lidocaine for neuropathic pain relief” Pain 87(1): 7-17.) bupivacaine, phenyloin (See, Jensen, T. S. (2002) “Anticonvulsants in neuropathic pain: rationale and clinical evidence” Eur J Pain 6 (Suppl A): 61-8), lamotrigine (See, Rozen, T. D. (2001) “Antiepileptic drugs in the management of cluster headache and trigeminal neuralgia” Headache 41 Suppl 1: S25-32 and Jensen, T. S. (2002) “Anticonvulsants in neuropathic pain: rationale and clinical evidence” Eur J Pain 6 (Suppl A): 61-8), and carbamazepine (See, Backonja, M. M. (2002) “Use of anticonvulsants for treatment of neuropathic pain” Neurology 59(5 Suppl 2): S14-7), have been shown to be useful attenuating pain in humans and animal models.
Hyperalgesia (extreme sensitivity to something painful) that develops in the presence of tissue injury or inflammation reflects, at least in part, an increase in the excitability of high-threshold primary afferent neurons innervating the site of injury. Voltage sensitive sodium channels activation is critical for the generation and propagation of neuronal action potentials. There is a growing body of evidence indicating that modulation of NaV currents is an endogenous mechanism used to control neuronal excitability (See, Goldin, A. L. (2001) “Resurgence of sodium channel research” Annu Rev Physiol 63: 871-94.). Several kinetically and pharmacologically distinct voltage-gated sodium channels are found in dorsal root ganglion (DRG) neurons. The TTX-resistant current is insensitive to micromolar concentrations of tetrodotoxin, and displays slow activation and inactivation kinetics and a more depolarized activation threshold when compared to other voltage-gated sodium channels. TTX-resistant sodium currents are primarily restricted to a subpopulation of sensory neurons likely to be involved in nociception. Specifically, TTX-resistant sodium currents are expressed almost exclusively in neurons that have a small cell-body diameter; and give rise to small-diameter slow-conducting axons and that are responsive to capsaicin. A large body of experimental evidence demonstrates that TTX-resistant sodium channels are expressed on C-fibers and are important in the transmission of nociceptive information to the spinal cord.
Intrathecal administration of antisense oligo-deoxynucleotides targeting a unique region of the TTX-resistant sodium channel (NaV1.8) resulted in a significant reduction in PGE2-induced hyperalgesia (See, Khasar, S. G., M. S. Gold, et al. (1998) “A tetrodotoxin-resistant sodium current mediates inflammatory pain in the rat” Neurosci Lett 256(1): 17-20). More recently, a knockout mouse line was generated by Wood and colleagues, which lacks functional NaV1.8. The mutation has an analgesic effect in tests assessing the animal's response to the inflammatory agent carrageenan (See, Akopian, A. N., V. Souslova, et al. (1999) “The tetrodotoxin-resistant sodium channel SNS has a specialized function in pain pathways” Nat Neurosci 2(6): 541-8.). In addition, deficit in both mechano- and thermoreception were observed in these animals. The analgesia shown by the Nav1.8 knockout mutants is consistent with observations about the role of TTX-resistant currents in nociception.
Immunohistochemical, in-situ hybridization and in-vitro electrophysiology experiments have all shown that the sodium channel NaV1.8 is selectively localized to the small sensory neurons of the dorsal root ganglion and trigeminal ganglion (See, Akopian, A. N., L. Sivilotti, et al. (1996) “A tetrodotoxin-resistant voltage-gated sodium channel expressed by sensory neurons” Nature 379(6562): 257-62.). The primary role of these neurons is the detection and transmission of nociceptive stimuli. Antisense and immunohistochemical evidence also supports a role for NaV1.8 in neuropathic pain (See, Lai, J., M. S. Gold, et al. (2002) “Inhibition of neuropathic pain by decreased expression of the tetrodotoxin-resistant sodium channel, NaV1.8” Pain 95(1-2): 143-52, and Lai, J., J. C. Hunter, et al. (2000) “Blockade of neuropathic pain by antisense targeting of tetrodotoxin-resistant sodium channels in sensory neurons” Methods Enzymol 314: 201-13.). NaV1.8 protein is upregulated along uninjured C-fibers adjacent to the nerve injury. Antisense treatment prevents the redistribution of NaV1.8 along the nerve and reverses neuropathic pain. Taken together the gene-knockout and antisense data support a role for NaV1.8 in the detection and transmission of inflammatory and neuropathic pain.
In neuropathic pain states there is a remodeling of Na channel distribution and subtype. In the injured nerve, expression of NaV1.8 and NaV1.9 are greatly reduced whereas expression of the TTX sensitive subunit NaV1.3 is significantly upregulated in animal models of neuropathic pain (See, Dib-Hajj, S. D., J. Fjell, et al. (1999) “Plasticity of sodium channel expression in DRG neurons in the chronic constriction injury model of neuropathic pain.” Pain 83(3): 591-600 and Kim, C. H., Youngsuk, O., et al. (2001) “The changes in expression of three subtypes of TTX sensitive sodium channels in sensory neurons after spinal nerve ligation”. Mol. Brain Res. 95:153-61.) The timecourse of the increase in NaV1.3 parallels the appearance of allodynia in animal models subsequent to nerve injury. Up-regulation of Nav1.3 transcription is also observed in a rat model of diabetic neuropathy. (See, Craner, M. J., Klein, J. P. et al. (2002) “Changes of sodium channel expression in experimental painful diabetic neuropathy.” Ann Neurol. 52(6): 786-92. The biophysics of the NaV1.3 channel is distinctive in that it shows very fast repriming after inactivation following an action potential. This allows for sustained rates of high firing as is often seen in the pathophysiological activity accompanying neuropathic pain (See, Cummins, T. R., F. Aglieco, et al. (2001) “Nav1.3 sodium channels: rapid repriming and slow closed-state inactivation display quantitative differences after expression in a mammalian cell line and in spinal sensory neurons” J Neurosci 21(16): 5952-61.). Human NaV1.3 channel proteins are expressed in the central and peripheral systems of man. (See, Chen, Y. H., Dale, T. J., et al. (2000) “Cloning, distribution and functional analysis of the type III sodium channel from human brain.” Eur. J. Neurosci. 12: 4281-89). Furthermore, in the periphery, NaV1.3 channel proteins are detectable in injured but not uninjured human nerves indicating that NaV1.3 plays important physiological roles under pathophysiological conditions in humans as well. Given the strong correlation between increased NaV1.3 channel expression and neuronal hyperexcitability, inhibitors of NaV1.3 channels, and in particular selective ones, might therefore provide efficacious therapeutic agents with less-severe side effects than nonselective Na_+ channel inhibitors in the treatment of painful neuropathies. Similarly, NaV1.3 overexpression may also be associated with increased epipleptic neuronal activity as it is significantly upregulated in hippocampal pyramidal neurons of epileptic humans (See, Whitaker, W. R. J., Faull, M., et al. (2001) “Changes in the mRNAs encoding voltage-gated sodium channel types II and III in human epileptic hippocampus.” Neurosci. 106(2): 275-285.); inhibitors with some selectivity against Nav1.3 could also be particularly attractive anticonvulsants and neuroprotectants.
NaV1.9 is similar to NaV1.8 as it is selectively localized to small sensory neurons of the dorsal root ganglion and trigeminal ganglion (See, Fang, X., L. Djouhri, et al. (2002). “The presence and role of the tetrodotoxin-resistant sodium channel Na(v)1.9 (NaN) in nociceptive primary afferent neurons.” J Neurosci 22(17): 7425-33.). It has a slow rate of inactivation and left-shifted voltage dependence for activation (See, Dib-Hajj, S., J. A. Black, et al. (2002) “NaN/Nav1.9: a sodium channel with unique properties” Trends Neurosci 25(5): 253-9.). These two biophysical properties allow NaV1.9 to play a role in establishing the resting membrane potential of nociceptive neurons. The resting membrane potential of NaV1.9 expressing cells is in the −55 to −50 mV range compared to −65 mV for most other peripheral and central neurons. This persistent depolarization is in large part due to the sustained low-level activation of NaV1.9 channels. This depolarization allows the neurons to more easily reach the threshold for firing action potentials in response to nociceptive stimuli. Compounds that block the NaV1.9 channel may play an important role in establishing the set point for detection of painful stimuli.
In chronic pain states, nerve and nerve ending can become swollen and hypersensitive exhibiting high frequency action potential firing with mild or even no stimulation. These pathologic nerve swellings are termed neuromas and the primary Na channels expressed in them are NaV1.8 and NaV1.7 (See, Kretschmer, T., L. T. Happel, et al. (2002) “Accumulation of PN1 and PN3 sodium channels in painful human neuroma-evidence from immunocytochemistry” Acta Neurochir (Wien) 144(8): 803-10; discussion 810.). NaV1.6 and NaV1.7 are also expressed in dorsal root ganglion neurons and contribute to the small TTX sensitive component seen in these cells. NaV1.7 in particular my therefore be a potential pain target in addition to it's role in neuroendocrine excitability (See, Klugbauer, N., L. Lacinova, et al. (1995) “Structure and functional expression of a new member of the tetrodotoxin-sensitive voltage-activated sodium channel family from human neuroendocrine cells” Embo J 14(6): 1084-90).
NaV1.1 (& Sugawara, T., E. Mazaki-Miyazaki, et al. (2001) “Nav1.1 mutations cause febrile seizures associated with afebrile partial seizures.” Neurology 57(4): 703-5.) and NaV1.2 (See, Sugawara, T., Y. Tsurubuchi, et al. (2001) “A missense mutation of the Na+ channel alpha II subunit gene Na(v)1.2 in a patient with febrile and afebrile seizures causes channel dysfunction” Proc Natl Acad Sci USA 98(11): 6384-9) have been linked to epilepsy conditions including febrile seizures. There are over 9 genetic mutations in NaV1.1 associated with febrile seizures (See, Meisler, M. H., J. A. Kearney, et al. (2002) “Mutations of voltage-gated sodium channels in movement disorders and epilepsy” Novartis Found Symp 241: 72-81).
Antagonists for NaV1.5 have been developed and used to treat cardiac arrhythmias. A gene defect in NaV1.5 that produces a larger noninactivating component to the current has been linked to long QT in man and the orally available local anesthetic mexilitine has been used to treat this condition (See, Wang, D. W., K. Yazawa, et al. (1997) “Pharmacological targeting of long QT mutant sodium channels.” J Clin Invest 99(7): 1714-20).
Several Na channel blockers are currently used or being tested in the clinic to treat epilepsy (See, Moulard, B. and D. Bertrand (2002) “Epilepsy and sodium channel blockers” Expert Opin. Ther. Patents 12(1): 85-91.); acute (See, Wiffen, P., S. Collins, et al. (2000) “Anticonvulsant drugs for acute and chronic pain” Cochrane Database Syst Rev 3), chronic (See, Wiffen, P., S. Collins, et al. (2000) “Anticonvulsant drugs for acute and chronic pain” Cochrane Database Syst Rev 3, and Guay, D. R. (2001) “Adjunctive agents in the management of chronic pain” Pharmacotherapy 21(9): 1070-81), inflammatory (See, Gold, M. S. (1999) “Tetrodotoxin-resistant Na+ currents and inflammatory hyperalgesia.” Proc Natl Acad Sci USA 96(14): 7645-9), and neuropathic pain (See, Strichartz, G. R., Z. Zhou, et al. (2002) “Therapeutic concentrations of local anaesthetics unveil the potential role of sodium channels in neuropathic pain” Novartis Found Symp 241: 189-201, and Sandner-Kiesling, A., G. Rumpold Seitlinger, et al. (2002) “Lamotrigine monotherapy for control of neuralgia after nerve section” Acta Anaesthesiol Scand 46(10): 1261-4); cardiac arrhythmias (See, An, R. H., R. Bangalore, et al. (1996) “Lidocaine block of LQT-3 mutant human Na+ channels” Circ Res 79(1): 103-8, and Wang, D. W., K. Yazawa, et al. (1997) “Pharmacological targeting of long QT mutant sodium channels” J Clin Invest 99(7): 1714-20); neuroprotection (See, Taylor, C. P. and L. S. Narasimhan (1997) “Sodium channels and therapy of central nervous system diseases” Adv Pharmacol 39: 47-98) and as anesthetics (See, Strichartz, G. R., Z. Zhou, et al. (2002) “Therapeutic concentrations of local anaesthetics unveil the potential role of sodium channels in neuropathic pain.” Novartis Found Symp 241: 189-201).
Voltage-gated calcium channels are membrane-spanning, multi-subunit proteins that open in response to membrane depolarization, allowing Ca entry from the extracellular milieu. Calcium channels were initially classified based on the time and voltage-dependence of channel opening and on the sensitivity to pharmacological block. The categories were low-voltage activated (primarily T-type) and high-voltage activated (L, N, P, Q or R-type). This classification scheme was replaced by a nomenclature based upon the molecular subunit composition, as summarized in Table I (Hockerman, G. H., et. al. (1997) Annu. Rev. Pharmacol. Toxicol. 37: 361-96; Striessnig, J. (1999) Cell. Physiol. Biochem. 9: 242-69). There are four primary subunit types that make up calcium channels—α1, α2δ, β and γ (See, e.g., De Waard et al. Structural and functional diversity of voltage-activated calcium channels. In Ion Channels, (ed. T. Narahashi) 41-87, (Plenum Press, New York, 1996)). The α1 subunit is the primary determinant of the pharmacological properties and contains the channel pore and voltage sensor (Hockerman, G. H., et. al. (1997) Annu. Rev. Pharmacol. Toxicol. 37: 361-96; Striessnig, J. (1999) Cell. Physiol. Biochem. 9: 242-69). Ten isoforms of the α1 subunit are known, as indicated in Table I. The α2δ subunit consists of two disulfide linked subunits, α2, which is primarily extracellular and a transmembrane δ subunit. Four isoforms of α2δ are known, α2δ-1, α2δ-2, α2δ-3 and α2δ-4. The β subunit is a non-glycosylated cytoplasmic protein that binds to the α1 subunit. Four isoforms are known, termed β1 to β4. The γ subunit is a transmembrane protein that has been biochemically isolated as a component of Cav1 and Cav2 channels. At least 8 isoforms are known (γ1 to γ8) (Kang, M. G. and K. P. Campbell (2003) J. Biol. Chem. 278: 21315-8). The nomenclature for voltage-gated calcium channels is based upon the content of the al subunit, as indicated in Table I. Each type of α1 subunit can associate with a variety of β, α2δ or γ subunits, so that each Cav type corresponds to many different combinations of subunits.
Cav Nomenclatureα1 subunitPharmacological nameCav1.1α1SL-typeCav1.2α1CL-typeCav1.3α1DL-typeCav1.4α1FCav2.1α1AP- or Q-typeCav2.2α1BN-typeCav2.3α1ER-typeCav3.1α1GT-typeCav3.2α1HT-typeCav3.3α1IT-type
Cav2 currents are found almost exclusively in the central and peripheral nervous system and in neuroendocrine cells and constitute the predominant forms of presynaptic voltage-gated calcium current. Presynaptic action potentials cause channel opening and neurotransmitter release is steeply dependent upon the subsequent calcium entry. Thus, Cav2 channels play a central role in mediating neurotransmitter release.
Cav2.1 and Cav2.2 contain high affinity binding sites for the peptide toxins ω-conotoxin-MVIIC and ω-conotoxin-GVIA, respectively, and these peptides have been used to determine the distribution and function of each channel type. Cav2.2 is highly expressed at the presynaptic nerve terminals of neurons from the dorsal root ganglion and neurons of lamina I and II of the dorsal horn (Westenbroek, R. E., et al. (1998) J. Neurosci. 18: 6319-30; Cizkova, D, et al. (2002) Exp. Brain Res. 147: 456-63). Cav2.2 channels are also found in presynaptic terminals between second and third order interneurons in the spinal cord. Both sites of neurotransmission are very important in relaying pain information to the brain.
Pain can be roughly divided into three different types: acute, inflammatory, and neuropathic. Acute pain serves an important protective function in keeping the organism safe from stimuli that may produce tissue damage. Severe thermal, mechanical, or chemical inputs have the potential to cause severe damage to the organism if unheeded. Acute pain serves to quickly remove the individual from the damaging environment. Acute pain by its very nature generally is short lasting and intense. Inflammatory pain, on the other hand, may last for much longer periods of time and its intensity is more graded. Inflammation may occur for many reasons including tissue damage, autoimmune response, and pathogen invasion. Inflammatory pain is mediated by a variety of agents that are released during inflammation, including substance P, histamines, acid, prostaglandin, bradykinin, CGRP, cytokines, ATP, and other agents (Julius, D. and A. I. Basbaum (2001) Nature 413 (6852): 203-10). The third class of pain is neuropathic and involves nerve damage arising from nerve injury or viral infection and results in reorganization of neuronal proteins and circuits yielding a pathologic “sensitized” state that can produce chronic pain lasting for years. This type of pain provides no adaptive benefit and is particularly difficult to treat with existing therapies.
Pain, particularly neuropathic and intractable pain is a large unmet medical need. Millions of individuals suffer from severe pain that is not well controlled by current therapeutics. The current drugs used to treat pain include NSAIDS, COX-2 inhibitors, opioids, tricyclic antidepressants, and anticonvulsants. Neuropathic pain has been particularly difficult to treat as it does not respond well to opioids until high doses are reached. Gabapentin is currently the most widely used therapeutic for the treatment of neuropathic pain, although it works in only 60% of patients and has modest efficacy. The drug is generally safe, although sedation is an issue at higher doses.
Validation of Cav2.2 as a target for the treatment of neuropathic pain is provided by studies with ziconotide (also known as ω-conotoxin-MVIIA), a selective peptide blocker of this channel (Bowersox, S. S., et al. (1996) J. Pharmacol. Exp. Ther. 279: 1243-9; Jain, K. K. (2000) Exp. Opin. Invest. Drugs 9: 2403-10; Vanegas, H. and H. Schaible (2000) Pain 85: 9-18). In man, intrathecal infusion of Ziconotide is effective for the treatment of intractable pain, cancer pain, opioid resistant pain, and neuropathic pain. The toxin has an 85% success rate for the treatment of pain in humans with a greater potency than morphine. An orally available antagonist of Cav2.2 should have similar efficacy without the need for intrathecal infusion. Cav2.1 and Cav2.3 are also in neurons of nociceptive pathways and antagonists of these channels could be used to treat pain.
Antagonists of Cav2.1, Cav2.2 or Cav2.3 should also be useful for treating other pathologies of the central nervous system that apparently involve excessive calcium entry. Cerebral ischaemia and stroke are associated with excessive calcium entry due to depolarization of neurons. The Cav2.2 antagonist ziconotide is effective in reducing infarct size in a focal ischemia model using laboratory animals, suggesting that Cav2.2 antagonists could be used for the treatment of stroke. Likewise, reducing excessive calcium influx into neurons may be useful for the treatment of epilepsy, traumatic brain injury, Alzheimer's disease, multi-infarct dementia and other classes of dementia, amyotrophic lateral sclerosis, amnesia, or neuronal damage caused by poison or other toxic substances.
Cav2.2 also mediates release of neurotransmitters from neurons of the sympathetic nervous system and antagonists could be used to treat cardiovascular diseases such as hypertension, cardiac arrhythmia, angina pectoris, myocardial infarction, and congestive heart failure.
Unfortunately, as described above, the efficacy of currently used sodium channel blockers and calcium channel blockers for the disease states described above has been to a large extent limited by a number of side effects. These side effects include various CNS disturbances such as blurred vision, dizziness, nausea, and sedation as well more potentially life threatening cardiac arrhythmias and cardiac failure. Accordingly, there remains a need to develop additional Na channel antagonists, and Ca channel antagonists preferably those with higher potency and fewer side effects.